Silicon carbide-based catalytic body and process for producing the same

ABSTRACT

The silicon carbide-based catalytic body of the present invention comprises:
         a porous body of given shape comprising a first bonded structure formed by bonding a large number of silicon carbide particles as an aggregate to each other in a state that a large number of fine pores are present, and   a catalyst containing an alkali metal and/or an alkaline earth metal, loaded on the porous body, characterized in that the catalyst is loaded via a crystalline coating film comprising an oxide and formed on at least part of the surfaces of the silicon carbide particles forming the first bonded structure. In the catalytic body, the catalyst such as NO x  occlusion catalyst or the like, loaded thereon can maintain its activity over a long period.

TECHNICAL FIELD

The present invention relates to a silicon carbide-based catalytic bodyhaving a catalyst loaded thereon, used for purification of automobileexhaust gas, as well as to a process for producing such a catalyticbody.

BACKGROUND ART

A porous honeycomb structure constituted by cell partition walls (ribs)forming a plurality of cells adjacent to each other (a cell group) andan outer wall surrounding and holding the outermost cells present at theperiphery of the cell group, is in wide use as a filter (a dieselparticulate filter, DPF) for capturing and removing the particulatesubstance contained in a dust-containing fluid (e.g. an exhaust gas fromdiesel engine) or as a catalyst carrier for loading thereon a catalystcomponent which purifies the harmful substance contained in an exhaustgas; and refractory silicon carbide (SiC) is being used as a matrixmaterial of the porous honeycomb structure.

As such a honeycomb structure, there is known, for example, a poroussilicon carbide-based catalyst carrier of honeycomb structure which isobtained by using, as a starting material, an impurity-containingsilicon carbide powder having a given specific surface area, forming thestarting material into a given shape, drying the formed body, and firingthe dried formed body in a temperature range of 1,600 to 2,200° C. (see,for example, JP-A-1994-182228).

In production of the above-mentioned porous silicon carbide-basedcatalyst carrier, the silicon carbide component vaporizes from thesurfaces of the silicon carbide powder (particles) and condenses at thecontacting areas (neck areas) of the particles; thereby, the neck areasgrow and a bonded state is obtained in the sintering (necking) byrecrystallization of the silicon carbide powder per se. However, thevaporization of silicon carbide requires a very high firing temperature,inviting a high cost, and the high-temperature firing of the materialhaving high thermal expansion coefficient causes a reduction in firingyield, which have been a problem.

Further, when it is attempted to produce, by the sintering byrecrystallization of the silicon carbide powder per se, a filter of highporosity, particularly a filter having a porosity of 50% or more, therehas been a problem in that the sintering mechanism does not functionsufficiently, thereby the growth of neck area is inhibited, consequentlythe filter produced has a low strength.

As a conventional technique for solving these problems, there weredisclosed a porous honeycomb structure containing refractory particlesas an aggregate, particularly silicon carbide and metallic silicon and aprocess for producing the porous honeycomb structure (see, for example,JP-A-2002-201082). However, this porous honeycomb structure has had aproblem in that the structure, when loaded with, for example, an alkalimetal (e.g. potassium, K) as an NO_(x) occlusion catalyst or the like,shows a rapid deactivation of catalyst. This is a problem which tends tooccur when a porous body containing silicon carbide as the maincomponent is used as a catalyst carrier, regardless of the shape of thecarrier.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above-mentionedproblems possessed by conventional techniques and aims at providing asilicon carbide-based catalytic body in which a catalyst such as NO_(x)occlusion catalyst or the like can maintain its activity over a longperiod, and a process for producing such a catalytic body.

According to the present invention, there is provided a siliconcarbide-based catalytic body comprising:

a porous body of given shape comprising a first bonded structure formedby bonding a large number of silicon carbide particles as an aggregateto each other in a state that a large number of fine pores are present;and

a catalyst containing an alkali metal and/or an alkaline earth metal,loaded on the porous body,

wherein the catalyst is loaded via a crystalline coating film comprisingan oxide and formed on at least part of the surfaces of the siliconcarbide particles forming the first bonded structure.

According to the present invention, there is also provided a siliconcarbide-based catalytic body comprising:

a porous body of given shape comprising a second bonded structure formedby bonding a large number of silicon carbide particles as an aggregateand metallic silicon as a binder in a state that a large number of finepores are present; and

a catalyst containing an alkali metal and/or an alkaline earth metal,loaded on the porous body,

wherein the catalyst is loaded via a crystalline coating film comprisingan oxide and formed on at least part of the surfaces of the siliconcarbide particles and/or the metallic silicon, forming the second bondedstructure.

In the present invention, it is preferred that the crystalline coatingfilm contains SiO₂ and further preferred that the crystalline coatingfilm comprises cristobalite and/or mullite.

In the present invention, it is also preferred that the given shape is ahoneycomb shape.

4. According to the present invention, there is also provided a processfor producing a silicon carbide-based catalytic body, the processcomprising:

forming a raw material mixture containing silicon carbide particles andmetallic silicon into a formed body of a given shape;

calcinating and firing the formed body;

heat treating the formed body in an oxygen-containing atmosphere; andthen loading, on the formed body, a catalyst containing an alkali metaland/or an alkaline earth metal,

to obtain a catalytic body comprising:

a porous body comprising a second bonded structure formed by bonding alarge number of the silicon carbide particles and the metallic siliconin a state that a large number of fine pores are present; and

the catalyst loaded on the porous body via a crystalline coating filmcomprising an oxide and formed on at least part of the surfaces of thesilicon carbide particles and/or the metallic silicon, forming thesecond bonded structure.

In the present invention, the heat treatment is conducted preferably ata temperature of 800 to 1,400° C. and the given shape is preferably ahoneycomb shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph showing the micro-structure (beforecatalyst loading) of the silicon carbide-based catalytic body of Example1.

FIG. 2 is an electron micrograph showing the micro-structure (beforecatalyst loading) of the silicon carbide-based catalytic body ofComparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors found that the phenomenon that a catalyst [e.g.potassium (K)] loaded on a porous honeycomb structure containing siliconcarbide and metallic silicon tends to be deactivated, occurs because thecatalyst is easily absorbed by and diffused in the amorphous silica(SiO₂) phase formed at the time of firing and heating of the poroushoneycomb structure which is a catalyst carrier. Therefore, it isconsidered that modification of the amorphous silica (SiO₂) phase canachieve a longer life of catalyst, and the present invention has beenmade.

The embodiments of the present invention are described below. However,the present invention is not restricted to the following embodiments andit should be construed that design change, improvement, etc. can be madeappropriately based on the ordinary knowledge of those skilled in theart without departing from the spirit of the present invention.

The first embodiment of the present invention is a silicon carbide-basedcatalytic body which comprises:

a porous body of given shape which is constituted by a first bondedstructure formed by bonding a large number of silicon carbide particlesas an aggregate to each other in a state that a large number of finepores are present, and

a catalyst containing an alkali metal and/or an alkaline earth metal,loaded on the porous body, characterized in that the catalyst is loadedvia a crystalline coating film composed of an oxide and formed on atleast part of the surfaces of the silicon carbide particles forming thefirst bonded structure. The first invention is described in detailbelow.

The silicon carbide-based catalytic body of the present embodimentcomprises a first bonded structure formed by bonding a large number ofsilicon carbide particles as an aggregate in a state that a large numberof fine pores are present in the structure. Therefore, the presentcatalytic body reflects the properties of silicon carbide which is aconstituent, and is superior in properties such as oxidation resistance,heat resistance and the like.

In the silicon carbide-based catalytic body of the present embodiment,at least part of the surfaces of the silicon carbide particles formingthe first bonded structure constituting the catalytic body, i.e. atleast part of the surfaces of the silicon carbide particles contactingwith the loaded catalyst containing an alkali metal and/or an alkalineearth metal is covered with a crystalline coating film comprising anoxide. This crystalline coating film comprising an oxide is a coatingfilm formed in place of the amorphous silica (SiO₂) phase which has beenformed on the surfaces of the silicon carbide particles in aconventional silicon carbide-based porous body. In the siliconcarbide-based catalytic body of the present embodiment, in which acatalyst containing an alkali metal and/or an alkaline earth metal isloaded via this crystalline coating film, the loaded catalyst is hardlyabsorbed and diffused and has a longer life.

Next, description is made on the second embodiment of the presentinvention. The second embodiment of the present invention is a siliconcarbide-based catalytic body which comprises:

a porous body of given shape which is constituted by a second bondedstructure formed by bonding a large number of silicon carbide particlesas an aggregate and metallic silicon as a binder in a state that a largenumber of fine pores are present, and

a catalyst containing an alkali metal and/or an alkaline earth metal,loaded on the porous body, characterized in that the catalyst is loadedvia a crystalline coating film composed of an oxide and formed on atleast part of the surfaces of the silicon carbide particles and/or themetallic silicon, all forming the second bonded structure. The secondembodiment is described in detail below.

The silicon carbide-based catalytic body of the present embodimentcomprises a second bonded structure formed by bonding silicon carbideparticles as an aggregate and metallic silicon as a binder in a statethat a large number of fine pores are present in the structure.Therefore, in production of the present catalytic body, sintering at arelatively low firing temperature is possible, the production cost islow, and the yield is high. Further, since metallic silicon is used forbonding silicon carbide particles which are refractory particles, thepresent catalytic body has a high thermal conductivity. Therefore, whenthe present catalytic body is used, for example, as a DPF and when theparticulate matter deposited thereon is burnt for regeneration of thefilter, there hardly appears a local temperature increase which damagesthe filter. The present catalytic body is also superior in propertiessuch as oxidation resistance, heat resistance and the like.

In the silicon carbide-based catalytic body of the present embodiment,at least part of the surfaces of the silicon carbide particles and themetallic silicon, forming the second bonded structure constituting thecatalytic body, i.e. at least part of the surfaces of the siliconcarbide particles and the metallic silicon, contacting with the loadedcatalyst containing an alkali metal and/or an alkaline earth metal iscovered with a crystalline coating film comprising an oxide. Thiscrystalline coating film comprising an oxide is a coating film formed inplace of the amorphous silica (SiO₂) phase which has been formed on thesurfaces of the silicon carbide particles and/or metallic silicon in aconventional silicon carbide-based porous body. In the siliconcarbide-based catalytic body of the present embodiment, in which acatalyst containing an alkali metal and/or an alkaline earth metal isloaded via this crystalline coating film, the loaded catalyst is hardlyabsorbed and diffused and has a longer life.

Incidentally, in the first and second embodiments of the presentinvention, “at least part of the surfaces of the silicon carbideparticles” and “at least part of the surfaces of the silicon carbideparticles and the metallic silicon” mean that the surfaces of thesilicon carbide particles and/or the metallic silicon may have areas notcovered with the crystalline coating film. However, it is especiallypreferred that all of the surfaces of the silicon carbide particlesand/or the metallic silicon, which are in contact with the loadedcatalyst containing an alkali metal and/or an alkaline earth metal, arecovered with the crystalline coating film.

In the first and second embodiments of the present invention, thecrystalline coating film comprising an oxide is preferred to containsilica (SiO₂) and more specifically be composed of cristobalite and ormullite. The crystalline coating film composed of these compounds ispreferred because a catalyst can be carried on it stably for a longerperiod and can be formed easily.

In the first and second embodiments of the present invention, it is alsopreferred that the porous body has a honeycomb shape (i.e. a honeycombstructure). The reason is that such a shape reflects the properties ofthe silicon carbide (constituent)-based porous body, shows superiorproperties in oxidation resistance, heat resistance, etc., and can beused under a high SV (space velocity) condition as a catalyst carrier.

As the kind of the alkali metal and/or alkaline earth metal contained inthe catalyst used in the present embodiments, there can be mentioned,for example, K, Li, Na and Cs (alkali metals) and Ca, Ba and Sr(alkaline earth metals). In the catalyst, there may also be containedordinarily, as a catalyst component, a noble metal such as Pt, Pd, Rh orthe like, besides a NO_(x) occlusion component (an alkali metal or analkaline earth metal). Such a noble metal allows the No present inexhaust gas to react with O₂ to generate NO₂, prior to NO_(x) occlusionby alkali metal or alkaline earth metal and, when the occluded NO_(x)has been released, allows the released NO_(x) to react with combustiblecomponents present in the exhaust gas to make the combustible componentsharmless. As the matrix material of the catalyst, there is preferablyused a heat-resistant inorganic oxide having large specific surfacearea, such as γ-Al₂O₃ or the like, for loading the above-mentionedNO_(x) occlusion component and noble metal in a highly dispersed state.

In order to load the catalyst containing an alkali metal and/or analkaline earth metal, via the crystalline coating film, there may beused a general method of catalyst loading, employed in loading acatalyst on a honeycomb structure. Incidentally, the method forproducing a honeycomb structure (a silicon carbide-based catalytic body)is described below.

Then, the third embodiment of the present invention is described. Thethird embodiment of the present invention is a process for producing asilicon carbide-based catalytic body, which comprises:

forming a raw material mixture containing silicon carbide particles andmetallic silicon, into a given shape,

calcinating and firing the formed body, followed by heat treatment in anoxygen-containing atmosphere, then

loading, on the resulting body, a catalyst containing an alkali metaland/or an alkaline earth metal, to obtain a catalytic body (a siliconcarbide-based catalytic body according to the second embodiment of thepresent invention). The catalyst comprises a porous constituted by asecond bonded structure formed by bonding a large number of the siliconcarbide particles and the metallic silicon in a state that a largenumber of fine pores are present, and the catalyst loaded on the porousbody via a crystalline coating film comprising an oxide and formed on atleast part of the surfaces of the silicon carbide particles and/or themetallic silicon, forming the second bonded structure. The thirdembodiment is described in detail below.

In producing the silicon carbide-based porous body, first, a rawmaterial mixture containing silicon carbide particles and metallicsilicon is prepared. To this raw material mixture may be added, asnecessary, a forming aid such as organic binder or the like. The siliconcarbide particles and the metallic silicon may contain a very smallamount of impurities such as Fe, Al, Ca and the like; however, they maybe used per se or may be used after purification by chemical treatmentsuch as washing with chemical agent. The raw material mixture is kneadedto obtain a clay for forming.

The clay is formed into a given shape such as honeycomb shape or thelike. The formed body is calcinated to remove the organic binder presenttherein (degreasing) to obtain a calcinated body. The calcination isconducted preferably at a temperature lower than the temperature atwhich metallic silicon melts. Specifically, the calcination may beconducted by holding the calcination temperature once at a given levelof about 150 to 700° C., or may be conducted by using a smalltemperature elevation rate of 50° C./hr or less in a given temperaturerange. When the calcination temperature is held once at a given level,holding may be made at one temperature level or at a plurality oftemperature levels depending upon the kind and amount of the organicbinder used; when holding is made at a plurality of temperature levels,holding of each temperature level may be the same or different. When asmall temperature elevation rate is used, such a temperature elevationrate may be used only in one temperature range or in a plurality oftemperature ranges; when the small temperature elevation rate is used ina plurality of temperature ranges, the temperature elevation rate ofeach range may be the same or different.

The calcinated body is fired, whereby a fired body can be obtained. Thefired body is constituted by the second bonded structure formed bybonding a large number of silicon carbide particles (which is a rawmaterial and an aggregate) and metallic silicon (which is a binder) in astate that fine pores are present. In order to form this second bondedstructure, it is necessary that the metallic silicon is softened duringthe firing. Since the melting point of the metallic silicon is 1,410°C., the firing is conducted preferably at a temperature of 1,410° C. orhigher. The optimum firing temperature is determined by the intendedmicrostructure and properties of the fired body to be obtained. With afiring temperature exceeding 1,600° C., vaporization of the metallicsilicon proceeds, making difficult the bonding of silicon carbideparticles via the metallic silicon; therefore, the firing temperature isappropriately 1,410 to 1,600° C., preferably 1,420 to 1,580° C.

Then, the fired body is heat-treated in an oxygen-containing atmosphere.By conducting this heat treatment, it is possible to cover at least partof the surfaces of the silicon carbide particles and/or the metallicsilicon, which form the second bonded structure constituting the firedbody, with a crystalline coating film comprising an oxide, for example,a coating film containing SiO₂, specifically a coating film composed ofcristobalite and/or mullite, whereby a silicon carbide-based porous body(which later becomes a catalyst carrier) can be obtained.

Incidentally, the heat treatment in an oxygen-containing atmosphere isconducted preferably at 800 to 1,400° C., more preferably at 1,100 to1,350° C. When the temperature is lower than 800° C., the degree ofcrystallinity of the crystalline coating film is insufficient; atemperature of higher than 1,400° C. is not preferred because it is nearthe melting point of metallic silicon, possibly having difficulty inkeeping the given shape.

Then, the porous body is loaded with a catalyst. As the matrix materialof the catalyst, a heat-resistant inorganic oxide of large specificsurface area, such as γ-Al₂O₃ or the like is preferably used because itcan carry the above-mentioned NO_(x) occlusion component and noble metalin a highly dispersed state. Incidentally, the specific example of themethod for loading a catalyst is described later.

The present invention is described specifically below by way ofExamples. However, the present invention is in no way restricted tothese Examples.

EXAMPLES 1 AND 2, COMPARATIVE EXAMPLES 1 AND 2

A SiC raw material powder having an average particle diameter of 47 μmand a Si powder having an average particle diameter of 5 μm werecompounded in a mass ratio of 80:20. To 100 parts by mass of theresulting powder mixture were added 6 parts by mass of methyl celluloseas an organic binder, 2.5 parts by mass of a surfactant and 24 parts bymass of water, followed by uniform mixing and kneading, to obtain a clayfor forming. The clay was formed by an extruder into a honeycomb shapeof 45 mm in outer diameter, 120 mm in length, 0.43 mm in partition wallthickness and 100 cells/in.² (16 cells/cm²) in cell density. The formedbody was then calcinated for degreasing at 500° C. for 5 hours and thenfired at 1,450° C. for 2 hours in a non-oxidizing atmosphere to obtain afired body.

The fired body was heat-treated under the conditions shown in Table 1(no heat treatment was made in Comparative Example 1) to produce asilicon carbide-based porous body having a honeycomb structure. Thecovered state of the porous body with a (crystalline) coating film wasobserved using an electron microscope and evaluated. The results areshown in Table 1. Incidentally, the evaluation was made based on thefollowing standard. A case in which the surfaces of the silicon carbide(SiC) particles and metallic silicon (Si) constituting the surface of aporous body to later contact with a catalyst had been coveredsufficiently, was rated as ⊚; a case in which the above surfaces hadbeen covered to some extent (about 50%), was rated as ◯; and a case inwhich the above surfaces had not been covered, was rated as X.

The crystal structure of the coating film observed above by an electronmicroscope was examined by X-ray diffraction. The results are shown inTable 1. Further, each porous body (honeycomb structure) was loaded withpotassium (K) as a catalyst component (a NO_(x) occlusion component)according to the method mentioned later, to produce siliconcarbide-based catalytic bodies (Examples 1 and 2, Comparative Examples 1and 2). The electron micrographs showing the microstructures (beforecatalyst loading) of the silicon carbide-based catalytic bodies ofExample 1 and Comparative Example 1 are shown in FIGS. 1 and 2.

(Preparation of Catalyst Raw Material and Production of CatalyticBodies)

A commercial γ-Al₂O₃ powder (specific surface area: 200 m²/g) wasimmersed in a mixed solution of an aqueous (NH₃)₂Pt(NO₂)₂ solution andan aqueous KNO₃ solution. They were stirred in a pot mill for 2 hours;then, the water was distilled to dryness. After dry grinding of theresidue, it was fired in an electric furnace at 600° C. for 3 hours toobtain a (platinum+potassium)-containing γ-alumina powder[(Pt+K)-predoped γ-Al₂O₃]. To this powder were added a commercial Al₂O₃sol and water, and wet grinding of the mixture was carried out in a potmill to prepare a catalyst raw material (a slurry for wash coat). Theratio of γ-Al₂O₃ to platinum (Pt) and potassium (K) was adjusted in thestage of immersion so that when the amount of the potassium (K) catalystloaded was 100 g per liter of honeycomb structure, the amount (mass) ofplatinum (Pt) became 30 g per cft of honeycomb structure (1.06 g perliter of honeycomb structure) and the amount (mass) of potassium (K)became 20 g per liter of honeycomb structure after final firing throughwash coat of slurry to the honeycomb structure. The addition amount ofthe Al₂O₃ sol was 5% by mass of the total Al₂O₃, in terms of Al₂O₃. Thewater was added appropriately so that the slurry had a viscosityallowing easy wash coating.

In the slurry for wash coat was immersed the above-produced honeycombstructure as a catalyst carrier; the superfluous solution in the cellsof the honeycomb structure were removed by blowing; then, the coatedhoneycomb structure was dried. Adjustment was made so that the amount(mass) of loaded potassium (K) after firing became 20 g per liter ofcatalyst carrier. When the intended loading amount was not achievedafter one time of immersion and drying, the steps of immersion anddrying were repeated until the intended loading amount was reached. Thepotassium (K)-loaded body was fired in an electric furnace at 600° C.for 1 hour to obtain each silicon carbide-based catalytic body.

(Evaluation of Suppressability of Potassium (K) Diffusion)

Each silicon carbide-based catalytic body was subjected to anaccelerated durability test in which the catalytic body was held at 850°C. for 30 hours with 10% by volume of water being allowed to be present.Before and after the test, the degree of dispersion (suppressability ofdiffusion) of potassium (K) was evaluated using the distribution ofpotassium (K) concentration (EDS map) measured by an energy dispersionspectroscopy. The results are shown in Table 1. Incidentally, theevaluation was made based on the degrees of diffusion of potassiumbefore and after the accelerated durability test. A case in which therewas substantially no potassium (K) diffusion and the distribution wasabout the same as before the test, was rated as A; a case in which therewas slight diffusion of potassium (K), was rated as B; a case in whichpotassium (K) remained slightly at the original positions but diffusedsubstantially, was rated as C; and a case in which potassium (K) did notremain substantially on the original positions, was rated as D. Theresults are shown in Table 1.

TABLE 1 Suppressa- Heat bility of SiC/Si treatment Condition Crystallinepotassium (mass Temp. Time of coating (K) ratio) (° C.) (h) coveringfilm diffusion Ex. 1 80/20 1200 24 ◯ Cristobalite B Ex. 2 80/20 1350 24⊚ Cristobalite A Comp. 80/20 — — X None C Ex. 1 Comp. 80/20 750 1000 ◯None D Ex. 2 (amorphous)(Results)

It became clear from Table 1 that when a given heat treatment wasapplied to a fired body, the surfaces of the silicon carbide (SiC)particles and metallic silicon (Si) constituting the surface of a porousbody to later contact with a catalyst could be covered with acrystalline coating film composed of cristobalite and the diffusion ofpotassium (K) could be suppressed effectively (Example 1). It becamealso clear also that by changing the conditions of heat treatment tofurther grow the crystalline coating film and sufficiently cover thesurface of the porous body to later contact with the catalyst, thediffusion of potassium (K) could be suppressed more effectively (Example2).

INDUSTRIAL APPLICABILITY

As described above, in the silicon carbide-based catalytic body of thepresent invention, the catalyst is loaded via a crystalline coating filmcomprising an oxide and formed on at least part of the silicon carbideparticles constituting a first bonded structure; therefore, a catalystsuch as NO_(x) occlusion catalyst or the like, loaded on the presentcatalytic body can maintain its activity over a long period.

Also in the silicon carbide-based catalytic body of the presentinvention, the catalyst is loaded via a crystalline coating filmcomposed of an oxide and formed on at least part of the silicon carbideparticles and/or metallic silicon, constituting a second bondedstructure; therefore, a catalyst such as NO_(x) occlusion catalyst orthe like, loaded on the present catalytic body can maintain its activityover a long period.

According to the process of the present invention for producing asilicon carbide-based catalytic body, since the process employs givenproduction steps and given production conditions, there can be easilyproduced a catalytic body in which a catalyst is loaded via acrystalline coating film composed of an oxide and formed on at leastpart of the surfaces of silicon carbide particles and/or metallicsilicon, forming a given bonded structure.

1. A silicon carbide-based catalytic body comprising: a porous body ofgiven shape comprising a first bonded structure formed by bonding alarge number of silicon carbide particles as an aggregate to each otherin a state that a large number of fine pores are present; and a catalystcontaining an alkali metal and/or an alkaline earth metal, loaded on theporous body, wherein the catalyst is loaded via a crystalline coatingfilm comprising an oxide and formed on at least part of the surfaces ofthe silicon carbide particles forming the first bonded structure.
 2. Asilicon carbide-based catalytic body according to claim 1 wherein thecrystalline coating film contains SiO₂.
 3. A silicon carbide-basedcatalytic body according to claim 2, wherein the crystalline coatingfilm comprises cristobalite and/or mullite.
 4. A silicon carbide-basedcatalytic body according to claim 1, wherein the given shape of theporous body is a honeycomb shape.
 5. A silicon carbide-based catalyticbody comprising: a porous body of given shape comprising a second bondedstructure formed by bonding a large number of silicon carbide particlesas an aggregate and metallic silicon as a binder in a state that a largenumber of fine pores are present; and a catalyst containing an alkalimetal and/or an alkaline earth metal, loaded on the porous body, whereinthe catalyst is loaded via a crystalline coating film comprising anoxide and formed on at least part of the surfaces of the silicon carbideparticles and/or the metallic silicon, forming the second bondedstructure.
 6. A silicon carbide-based catalytic body according to claim5, wherein the crystalline coating film contains SiO₂.
 7. A siliconcarbide-based catalytic body according to claim 6, wherein thecrystalline coating film comprises cristobalite and/or mullite.
 8. Asilicon carbide-based catalytic body according to claim 5, wherein thegiven shape of the porous body is a honeycomb shape.
 9. A process forproducing a silicon carbide-based catalytic body, the processcomprising: forming a raw material mixture containing silicon carbideparticles and metallic silicon into a formed body of a given shape;calcinating and firing the formed body; heat treating the formed body inan oxygen-containing atmosphere; and then loading, on the formed body, acatalyst containing an alkali metal and/or an alkaline earth metal, toobtain a catalytic body comprising: a porous body comprising a secondbonded structure formed by bonding a large number of the silicon carbideparticles and the metallic silicon in a state that a large number offine pores are present; and the catalyst loaded on the porous body via acrystalline coating film comprising an oxide and formed on at least partof the surfaces of the silicon carbide particles and/or the metallicsilicon, forming the second bonded structure.
 10. A process forproducing a silicon carbide-based catalytic body according to claim 9,wherein the heat treatment is conducted at a temperature of 800 to1,400° C.
 11. A process for producing a silicon carbide-based catalyticbody according to claim 9, wherein the given shape is a honeycomb shape.